1. Field of the Invention
The present invention generally relates to DRAM fabrication and more particularly to self-aligned contact etching.
2. Background Description
A self-aligned contact (SAC) etch is one of the most difficult reactive ion etch (RIE) processes in a DRAM fabrication sequence. It is a three step process utilizing different etch chemistries. Step one is a non-selective or traditional oxide etch of silicon dioxide to the gate cap Nitride. The next step involves the etching of borophosphorous silicate glass (BPSG) with a very high selectivity to Nitride liner and gate cap Nitride, which tests the limit of a RIE tool. Finally, in the third step the bottom liner is removed through to a silicon substrate so that an ohmic contact can be formed subsequently.
One problem with the current SAC and nitride liner process is that it requires high temperatures (720-800xc2x0 C.). A reduced thermal budget is preferred in future generations of DRAM devices to minimize dopant diffusion.
Better selectivity to nitride (liner) is needed for an improved bit-line shorts yield. However, there is also a need to thin the nitride liner as much as possible, for improved BPSG gap-fill. Hence, from the point of view of gap-fill by BPSG a thinner liner would be better, but for contact etch/RIE a thicker liner is better to relax selectivity requirements.
Currently, the contact nitride liner used for 0.175 xcexcm technology is Dichloro Silane (DCS) based LPCVD Nitride. Even though the selectivity to this liner during contact etch has greatly been improved due to new gas chemistries and processes that are continually being evaluated and introduced, it is still desirable to improve the core selectivity during RIE to improve shorts yield. Improvement in contact shorts yield is still one of the main issues facing many DRAM producing fabricators. Prior art has disclosed methods to improve this selectivity by exposure to CF4 after Nitride liner deposition. U.S. Pat. No. 5,935,873 to Spuler et al. discloses a method for forming a self-aligned contact in a semiconductor device incorporating carbon in a nitride layer. As disclosed, a portion of the nitride layer is carbonized to reduce the nitride etch rate. Specifically, the Spuler et al. reference discloses carbonizing only a top surface on the nitride. During etching processes, when this surface carbonization is broken through, undercutting during etch occurs rapidly. It was thought that CF4 polymer would enhance the selectivity to Nitride. However, the art does not mention or explain as to how difficult and expensive it may be to deposit a layer of CF4 or xe2x80x98diamond-like carbonxe2x80x99 (DLC), or whether it would even be feasible to deposit such a layer. In short, prior art did not show a practical method to improve selectivity to contact liner. To our knowledge, we are not aware of any developments whereby CF4 has been successfully incorporated into the said Nitride film to improve its selectivity to RIE etch.
It is therefore an object of the present invention to provide a low temperature carbon rich oxy-nitride liner process which has improved RIE (reactive ion etch) selectivity.
In this invention a practical method of improving the selectivity to contact liner is shown along with preliminary data to back up our claims. The main embodiment of this invention is the use of bis-tertbutylaminosilane (BTBAS or SiH2(NH(C4H9))2) precursor to deposit silicon oxynitride for contact liner application. However, other similar organosilane or organoaminosilane based precursors such as ditertiarybutyl silane may also be used. The object of this invention is the use of novel oxynitride precursors that when used would result in a precalculated amount of carbon content that is uniformly distributed in the final deposited film. BTBAS is used in the preferred embodiment. Further, the object is to use this film as a RIE barrier.
The basis of the invention hinges on two factors: 1) Use of BTBAS precursor to obtain a carbon rich nitride film and 2) deposition of oxy-nitride film as opposed to nitride film. The use of BTBAS precursor also allows the lowering of the deposition temperature from 720-800xc2x0 C. for process of record conditions to about 600xc2x0 C. Lowering of processing temperature minimizes diffusion of dopants. Such diffusion would undesirably change or perturb dopant profiles. The increased carbon concentration in such a film also provides better selectivity during SAC process and improved process window as explained below.